Gravity Indicators in the Near‐Infrared Spectra of Brown Dwarfs

We investigate the sensitivity to temperature and gravity of the strongabsorption features in the J- and K-band spectra of substellar objects. Wecompare the spectra of giants and young M dwarfs (of low gravity) to field Mand L dwarfs (of high gravity) and to model spectra from the Lyon group. Wefind that low-resolution spectra of M4 - M9 stars and young brown dwarfs at R ~350 and S/N > 70 can determine the spectral type to a precision of pm 1subtype, using the H2O and CO bands, and can measure the surface gravity to pm0.5 dex, using the atomic lines of KI and NaI. This result points toward thedevelopment of photometric spectral indices to separate low-mass members fromforeground and background objects in young clusters and associations. We alsoemphasize the complexity of the interpretation of the empirical quantities(e.g., spectral types) in terms of the physical variables (e.g., temperature,opacities) in the cool atmospheres of young brown dwarfs.Comment: Accepted to ApJ, 44 pages & 10 figures, preliminary results reported in astro-ph/020822